How open source Xen lost its footing

Open source Xen initially caused a stir in the industry, but a series of moves from Linux vendors has stunted the hypervisor’s adoption rate.

What happened to open-source Xen?

Five years ago, open source Xen shook up data centers with a new approach to virtualization. Called paravirtualization, Xen allowed the hypervisor and the virtual machine (VM) to communicate with one another, and it was available for free in all Linux distributions. Today, however, open source Xen has taken a backseat to other hypervisors -- most notably, VMware ESXi, Microsoft Hyper-V and KVM. What happened?

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In the days when open source Xen appeared in every Linux distribution, including Red Hat Enterprise Linux and SUSE Linux Enterprise Server, the hypervisor still had flaws -- mainly, the Linux kernel didn’t support it. As a result, open source Xen was bound to a specific Linux kernel. For hypervisor functionality, IT admins had to apply open Source Xen as a patch onto the mainstream kernel. They could not upgrade the kernel after the fact, because a kernel update would likely break the Xen functionality. Despite open source Xen’s great server virtualization features, in this way, it slowed down the adoption of other Linux functionality.

Linux vendors shy away from open source Xen
Another virtualization approach, called Kernel-based Virtual Machine (KVM) appeared around the same time as open source Xen. Instead of a large number of complex patches, IT admins implemented KVM with a few kernel modules, and the Linux kernel supported the hypervisor.

KVM started to become popular in 2008, incidentally the same year that Canonical Ltd. started to offer it as the default hypervisor in its Long Term Support (LTS) Ubuntu release, which targeted the corporate data center.

Red Hat Inc. made a more drastic statement when it released Red Hat Enterprise Linux (RHEL) 6. Previously, RHEL 5 featured the Xen hypervisor, but Red Hat removed all support for Xen in RHEL 6 and used KVM instead. Rather than offering open source Xen support, Red Hat provided conversion utilities to migrate Xen VMs to KVM.

The actions of Novell Inc., which owned SUSE Linux at the time, also hurt Xen’s adoption in data centers. In a matter of two years, Novell changed its server virtualization strategy. In 2008, at the annual BrainShare user conference, Novell confidently announced that SUSE Linux Enterprise Server, complete with the Xen hypervisor, would be an alternative to VMware ESX, and it even acquired PlateSpin to complete the offering.

But, in 2009, Novell’s enthusiasm for server virtualization waned and Xen virtualization wasn’t a priority. Insiders believe that Novell’s 2006 deal with Microsoft influenced the shift. In exchange for Novell offering insights into Xen virtualization, Microsoft began reselling vouchers for SUSE Linux Enterprise Server, becoming its biggest reseller.

Open source Xen on life support
Though enterprise Linux distributions turned their back on open source Xen, the hypervisor has not vanished completely. Citrix Systems Inc. bought XenSource, the company created by the founders of open source Xen. But Citrix released only part of the XenServer source code, with the ultimate goal of earning money off the platform.

Oracle VM is another virtualization platform based on Xen, but it hardly exists beyond the current circle of Oracle customers. Oracle VM is also not just a Xen hypervisor included in the Linux kernel, but a complete virtualization platform on its own.

In early 2011, long after the initial hype, open source Xen finally made it into the Linux kernel. Xen offers mature virtualization features, such as optimal virtualized OS support through paravirtualization, when compared to its open source opponent, KVM. Some people involved in Linux distributions have discussed making Xen the default hypervisor for Linux.

But Xen still faces an uphill battle. Ubuntu and Red Hat, two major Enterprise Linux players, have made it very clear that KVM is their hypervisor of choice. As such, if you insist on using open source Xen on a Linux platform, SUSE Linux Enterprise Server and Oracle are the only options. The open source world, following the lead of Red Hat and Ubuntu, has clearly opted for KVM.

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